The present invention relates to a printer, digital copier, facsimile apparatus or similar electrophotographic image forming apparatus. More particularly, the present invention relates to a developing method for causing a developer to form a magnet brush on a developer carrier in a developing region for developing a latent image formed on an image carrier, and a device for practicing the same.
A developing device for an image forming apparatus is operable with either one of a one-ingredient type developer, or toner, and a two-ingredient type developer or toner and magnetic carrier mixture. The two-ingredient type developer allows the frictional charging of its toner to be easily controlled, causes the toner to cohere little, and therefore allows toner transfer to be effectively controlled by, e.g., a bias, compared to the one-ingredient type developer. Further, the toner of the two-ingredient type developer does not have to contain a magnetic material or needs only a small amount of magnetic material if necessary to obviate, e.g., fog. Particularly, the two-ingredient type developer insures images with clear colors. Moreover, when a developer layer contacts an image carrier in the form of a magnet brush, it sharply rises and contacts the image carrier in a desirable manner. This is why the two-ingredient type developer is predominant over the one-ingredient type toner although its toner must be controlled in amount relative to the carrier.
The two-ingredient type developer, however, brings about the following problems. A one-dot line formed in a direction perpendicular to a direction of sheet conveyance, i.e., a horizontal line is thinned, compared to a line formed in the direction of sheet conveyance (thinning of a horizontal line hereinafter). The trailing edge of, e.g., a halftone portion in the direction of sheet conveyance is lowered in density or practically lost (omission of a trailing edge hereinafter). In light of this, Japanese Patent Laid-Open Publication No. 7-140730, for example, proposes to set the angle of the main pole of a magnet roller at the upstream side or to set up a preselected relation between a distance between a metering member and a developing sleeve and a distance between the developing sleeve and a photoconductive element. This kind of method should satisfy the following conditions (1) through (5):
(1) The main pole lies in a range of from 5° to 20° upstream of the closest position in a direction of developer conveyance;
(2) A doctor gap Hcut between the metering member and the developing sleeve or developer carrier is 0.25 mm to 0.75 mm;
(3) A development gap Dsd between the developing sleeve and the photoconductive element or image carrier is 0.30 mm to 0.80 mm;
(4) A ratio Dsd/Hcut is greater than 1.20, but smaller than 1.60; and
(5) A ratio of the moving speed Vs of the developing sleeve to the moving speed Vp of the photoconductive element is equal to or greater than 1.0, but equal to or smaller than 3.0.
The document mentioned above describes that when the conditions (1) through (5) are satisfied, a halftone portion or a solid portion is free from brush marks and the discontinuity of a fine line in a high copying speed range, achieving high, uniform density, and a clear-cut contour.
The method taught in the document, however, has the following problems left unsolved. As the ratio Dsd/Hcut shifts from 1, i.e., as the doctor gap Hcut becomes smaller than the development gap Dsd, the magnet brush between the developing sleeve and the photoconductive element becomes rough. The magnet brush therefore fails to uniformly contact the photoconductive element. Consequently, in a solitary dot image, in particular, in which dots with resolution of, e.g., 600 dpi (dots per inch) are recorded at the intervals of five to ten pixels, part of the dots is reduced in size or practically lost. This degrades the reproducibility and therefore tonality of a so-called high contrast portion. Further, as for a halftone image with image density ID ranging from 0.3 to 0.8, the irregular contact of the magnet brush aggravates granularity.
Japanese Patent Publication No. 2-59995 proposes to enhance a developing ability by bringing magnetic poles adjoining a main magnetic pole closer to the main magnetic pole. This document describes that although such a configuration lowers the density of a horizontal line (thinning of a horizontal line), this problem can be coped with by lowering the saturation magnetization of the carrier. However, when the saturation magnetization of the carrier is lowered, the deposition of the carrier is apt to occur. Should the amount of charge to deposit on the toner be reduced to avoid the deposition of the carrier, uncharged toner would increase and contaminate the background of an image.
Japanese Patent Laid-Open Publication No. 6-149063 teaches a non-contact type developing device using a two-ingredient type developer and arranging magnetic poles in such a manner as to prevent a magnet brush from contacting a photoconductive element. This developing device should satisfy the following conditions (1) through (3):
(1) The magnetic pole arrangement is set between a pair of N and S poles;
(2) The N and S poles make an angle of 40° to 70° therebetween, and each has a flux density of 500 mT or above; and
(3) A magnet angle between a position where an image carrier and a magnet brush roll are closest to each other and the center between the poles is between 0° and one-tenth of the angle between the poles, and a developing position is located between the poles of the magnet.
The document describes that when the conditions (1) through (3) are satisfied, a high quality image is attainable that is free from fog ascribable to the deposition of a carrier on the background of the image carrier and local omission around the deposited carrier. However, an electric field for development available with non-contact type development using the two-ingredient type developer is too weak to enhance a developing ability.
Generally, the absolute value of a difference between the charge potential of a photoconductive element and a bias for development, i.e., so-called background potential is related to the thinning of a horizontal line and the omission of a trailing edge. In the conventional developing device, the above defects can be reduced to an acceptable level if the background potential is reduced to, e.g., about 100 V or about 50 V. Such a low background potential, however, brings about background contamination or fog. This is particularly true in a hot, humid environment.
On the other hand, Japanese Patent application No. 11-318490 discloses an image forming apparatus capable of obviating the thinning of a horizontal line and the omission of a trailing edge. Further, this apparatus prevents solitary dots from being lost due to the irregular contact of a magnet brush and frees a halftone image from granularity. In addition, the apparatus obviates the deposition of the carrier to thereby maintain a high developing ability. However, a problem with this apparatus is that the magnet brush actively moves in a small gap between an image carrier and a developer carrier, causing the carrier to fly about during development and deposit on the image carrier as well as on the other members. Consequently, the image carrier is apt to convey the carrier to an image transfer position. The carrier therefore prevents toner around the carrier from being transferred to a paper sheet or similar recording medium, resulting in a defective image. Moreover, if the carrier is transferred to the paper sheet, it simply constitutes an impurity in the resulting image because it is not fixed on the paper sheet.
Granularity often appears in images, particularly halftone images, output by the conventional image forming apparatuses. Granularity is one of major causes that lower image quality.
It is a common practice with an image forming apparatus to maintain the density of a toner image by forming a particular toner image (patch hereinafter) on a photoconductive element or an intermediate image transfer body and sense the density of the patch with a density sensor. The sensed density is fed back in order to adequately control the quantity of light for exposure or a bias for development. With this scheme, it is possible to maintain images constant despite, e.g., the varying environment and aging. Because the patch is not expected to be printed by the image forming apparatus, it is simply collected by cleaning means after the sensing of the density. This wastefully consumes toner and needs replenishment of extra toner while increasing the amount of waste toner collected. Further, the patch is formed in a non-image portion not corresponding to a paper sheet and therefore smears an image transfer belt, an image transfer roller, an intermediate image transfer belt, and members contacting them. The toner deposited on such members is transferred to the back or the background of the resulting print, making the print defective.
Moreover, the toner forming the patch flies about to contaminate the density sensor. Particularly, a light-sensitive portion forming part of the sensor, which adjoins the patch in order to enhance accuracy, is contaminated more than the other member. The toner deposited on the sensor lowers the output of the sensor and thereby obstructs the accurate sensing of density. To solve this problem, Japanese Patent Laid-Open Publication No. 11-202696 proposes to inform the operator of the contamination of the sensor by using extra means for sensing it. This method, however, is not a drastic solution because it needs the extra means and requires the operator or a serviceman to clean the sensor. In addition, the patch size should be as small as possible because the contamination derived from the path lowers sensing accuracy.
In the conventional developing system using a magnet brush, a developing condition for increasing image density and a developing condition for rendering a low contrast image desirable are contradictory to each other. It is therefore difficult to improve both of a high density portion and a low density portion at the same time. More specifically, to increase image density, the gap between the image carrier and the developing sleeve (development gap) may be reduced, or the width of the developing region may be increased. On the other hand, to render a low contrast portion desirable, the development gap may be increased, or the developing region may be reduced. The two developing conditions are therefore contradictory. It is generally considered to be difficult to achieve an attractive image by satisfying the two conditions over the entire density range.
An increase in development gap serves to reduce the frictional force of the magnet brush acting on the image carrier, thereby reducing the omission of a trailing edge and promoting the faithful reproduction of a horizontal line. However, a greater development gap enhances an edge effect during development, i.e., develops solitary dots in a greater size than expected, thickens lines, enhances a portion around a solid image portion or a halftone image portion or causes the outside of such an image portion to be lost. As a result, sophisticated control over tonality reproduction is required. A small development gap reduces the edge effect and frees an image from noticeable granularity. A small development gap, however, intensifies the frictional force of the magnet brush and thereby aggravates the omission of a trailing edge and that of dots while obstructing the reproduction of a horizontal line. The resulting image is therefore noticeably dependent on direction.
As for an electrophotographic image forming apparatus, there is an increasing demand for higher resolution and higher tonality. A problem in this respect is that high pixel density reduces the individual pixel relative to the spot diameter of a beam to issue from an exposing unit, preventing sufficient tonality from being achieved.
Tonality is dependent on the beam spot diameter, as well known in the art. A large beam spot diameter relative to pixel density degrades the reproducibility of a low density portion or highlight portion. This is because when a solitary dot is written, a latent image representative of it is shallow due to low exposure energy density, making reproduction unstable. On the other hand, in a high density portion, nearby pixels are exposed in such a manner as to overlap each other with the result that image density rapidly saturates relative to a density area ratio, causing gamma to rise, i.e., lowering tonality. While the quantity of light for a dot may be increased to reproduce a solitary dot, the solitary dot increases in size and further aggravates tonality.
Therefore, to enhance resolution while maintaining tonality, the beam spot diameter must be reduced in accordance with pixel density. In laser optics, for example, the beam spot diameter can be reduced if the wavelength of a laser beam is reduced or if the numerical aperture (NA) of an f/θ lens is increased. On the other hand, in an LED (Light Emitting Diode) array or similar solid state optics, use may be made of a selfoc lens array (SLA), or LEDs may be reduced in size.
Today, high resolution and high tonality, which have been difficult to achieve with conventional image forming apparatuses due to accuracy and cost problems, are available with may products. However, as for exposure using a beam whose spot diameter is equivalent to a pixel size, tonality is not sufficient when it comes to recent, high density images. Particularly, reproducibility tends to decrease in a highlight portion with an increase in recording density for the following reasons. As for a latent image representative of solid dots, a charge distribution corresponding to a small dot size is attainable. However, during development, the edge effect renders the dots in a size greater than the target size. Further, the magnet brush with countercharge after development rubs itself against the toner image, so that the toner is returned to the developing roller. This aggravates irregularity in the area of the dot and thereby lowers the reproducibility of a highlight portion.
Moreover, when the recording density is as high as 1,200 dpi, solitary dots are further reduced in size and cannot be easily formed by development. In addition, the reproducibility of a highlight portion is lowered.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-160725 and 2000-305360.